time is IllS and the access times required from stable address and from read (RD) or write (WR) command are approximately 600ns and 250ns respectively.
2.1.2.3 TIMER/EVENT COUNTERS
The 8051 contains two 16-bit counters for measuring time intervals, measuring pulse widths, counting events and generating precise, periodic interrupt requests. Each can be programmed independently to operate similar to an 8048 8-bit timer with divide by 32 prescaler or 8-bit counter with divide by 32 prescaler (Mode 0), as a 16-bit time-interval or event counter (Mode I), or as an 8-bit time-interval or event counter with automatic reload upon overflow (Mode 2).
Additionally, counter 0 can be programmed to a mode that divides it into one 8-bit time-interval or event counter and one 8-bit time-interval counter (Mode 3).
When counter 0 is in Mode 3, counter I can be programmed to any of the three aforementioned modes, although it cannot set an interrupt request flag or generate an interrupt. This mode is useful because counter I's over- flow can be used to pulse the serial port's transmission-rate generator. Along with their multiple operating modes and 16-bit precision, the counters can also handle very high input frequencies. These range from 0.1 MHzto 1.0 MHz (for 1.2 MHz to 12 MHz crystal) when programmed for an input that is a division by 12 of the oscillator frequency and from 0 Hz to an upper limit of 50 KHz to 0.5 MHz (for 1.2 MHz to 12 MHz crystal) when programmed for external inputs. Both· internal and external inputs can be gated to the counter by a second external source for directly measuring pulse widths.
The counters are started and stopped under software control. Each counter sets its interrupt request flag when it overflows from all ones to aU zeros (or auto-reload value). The operating modes and input sources are summarized in Figures 2.4A and 2.4B. The effects of the configuration flags and the status flags are shown in Figures 2.5A and 2.5B.
COUNTER/TIMER RUN
GATE
INTO --~=-~l:'~>-"t"--r-\--J====j~:J
TO------~
CRYSTAL
OSCILLATOR
| $ | OVERFLOW |
| ~_--I~(INTERRUPT |
| EXTERNAL-. |
| SOURCE | REQUEST) |
| | FLAG 0 |
•8048 TIMER/COUNTER
•16-BIT TIMER/COUNTER
•8-BIT AUTO-RELOAD TIMER/COUNTER
CRYSTAL
°ii~O,.R__~~~~~__~
| | OVERFLOW |
EXTSOEURRNCALE - . | 1---4t----I~ (INTERRUPT |
| | REQUEST) |
| | FLAG 1 |
• | 8048 TIMER/COUNTER | |
• | 16-BIT TIMER/COUNTER | PULSE TO |
• | 8-BIT AUTO-RELOAD TIMER/COUNTER |
| | SERIAL PORT |
Figure 2.4.A. Timer/Event Counter
| | Modes 0, 1, and 2 | |
| CRYSTAL | CRYSTAL | |
| OSCilLATOR | OSCILLATOR | |
EXTERNAL ___$ | | OVERFLOW |
| (INTERRUPT |
| REQUEST) |
| SOURCE | | FLAG 1 |
| | OVERFLOW |
| | |
| | L--____--I~(INTERRUPT |
| | | REQUEST) |
• | 8-BIT TIMER/COUNTER | • 8-BIT TIMER | FLAG 0 |
| CRYSTAL | | |
| OSCilLATOR | | |
| $ | | |
EX;5~:~~'" | | |
• | 8048 TIMER/COUNTER | | PULSE TO |
• | 16-BIT TIMER/COUNTER | |
• | 8-BIT AUTO-RELOAD TIMER/COUNTER | SERIAL PORT |
Figure 2.4.8. Timer/Event Counter Mode 3
INTERRUPT
REQUEST
COUNTER 0
MODE 0: 8-BIT TIMER WITH PRESCALER/8-BIT COUNTER
WITH PRESCAlER
>----fMODE 1: 16-BIT TIMER/COUNTER
MODE 2: 8-BIT AUTO-RELOAD TIMER/COUNTER
MODE 3: 8-BIT TIMER/COUNTER (TlO)
